The present invention relates primarily to methods of improving the efficiency, reducing environmental issues, and operational and capital costs, of desalination systems. More particularly, to desalination systems that distill brackish or ocean water.
Fresh water is a scant 2.5% of the total global water supply and 69% of that is represented by permanent snow and glaciers. The remaining 97.5% is saltwater. Since 1940, the amount of fresh water used by humanity has roughly quadrupled as the world population doubled. Given the finite nature of the earth's fresh water resources, such a quadrupling of worldwide water use probably cannot occur again. In many of the regions where the world population is growing most rapidly, the needed fresh water is not available. Desalination of seawater represents the best source of fresh water to satisfy future requirements.
However, present day desalination systems are energy intensive. For example, the newly constructed system in Carlsbad, Calif. is said to be the most energy efficient of any large scale desalination system in the USA at 3.6 kilowatts per cubic meter of water. It also desalinates only fifty percent of intake water, returning the remaining concentrated brine to the ocean. Returning concentrated brine solution to the ocean presents a continually escalating environmental hazard to the ocean ecosystem.
For desalination to be the source of fresh water to meet future requirements, it must be cost competitive with ground water sources and environmentally friendly.
The true cost of household fresh water is difficult to assess due to government subsidies, transfer cost and variations in local energy and labor cost. However, it is estimated that energy requirements for desalination should be in the range of about 2 to 2.5 kilowatts per cubic meter of fresh water to be competitive.
Another environmental issue involves seawater intakes that can only be addressed in connection with site location of the desalination system. However, there are intake methods such as subsurface, sand filters, subterranean, and beach wells that can solve most environment intake problems.
Throughout the world today, all desalination facilities combined produce about 38 million cubic meters (approx. 10 billion gallons) of desalinated water per day. These facilities basically utilize two technologies, membrane filter processes and thermal distillation processes. Of these processes, reverse osmosis (membrane filter process) and multi-stage flash distillation (thermal distillation process), make up and share about 80% of the world market.
Reverse osmosis uses high pressure pumps to force fresh water through a semi-permeable membrane, leaving the dissolved solids behind. This process requires seawater pretreatment, an electrical power source, chemical post-treatment and annual membrane replacement.
Multi-stage flash (MSF) involves introducing heated seawater into multiple, reduced pressure chambers that cause a portion of the water to instantly flash (boil) into water vapor. The vapor is then condensed into distilled water. This process requires an energy source for heating the seawater as well as control functions.
Both technologies are energy intensive, and both convert about 50% of the input seawater into fresh drinkable water, discharging the remaining brine solution back into the ocean, which results in an ever increasing environmental problem.
The past decade has seen a huge increase in research and development in desalination projects around the world utilizing improved technologies, resulting in improved efficiency and reduced capital costs, such as low temperature flash desalination. Numerous patents have been granted disclosing designs that improve efficiency. A large number of these patents involve the “flash desalination” of water at low, near ambient temperatures in an effort to reduce energy requirements. Although seawater can be evaporated at low temperatures by decreasing pressure (partial vacuum), the decreasing temperature results in an exponential decrease in the Vapor Saturation Density. Therefore, large quantities of vapor must be transferred to recover significant quantities of distilled liquid, which places much higher energy and costs requirements upon the system.
For example, at 40° C. (104° F.), saturated vapor density is 51.1 grams per cubic meter (0.00319 pounds per cubic feet). At 110° C. (230° F.), saturated vapor density is 850 grams per cubic meter (0.05306 pounds per cubic feet). The result is that a system that is to produce 100 cubic meters (26,417 gal) of fresh water per day at a temperature of 40° C. must transfer vapor at a rate of more than 1359 cubic meters per min, whereas at 110° C. it would only need to transfer 81.7 cubic meters per min.
Despite the inventions, research, developments and improvements, present day seawater desalination processes continue to be an intensive fossil energy consumer that escalates desalination cost from to 5 times greater than ground water supplies.
The desalination industry has publicized that the minimum energy requirement to desalinate 3.5% seawater is 860 watts per cubic meter. A true statement, but somewhat misleading in that the process does require 860 watts per cubic meter to remove the dissolved solids. However, desalination is a reversible process; therefore, the energy used for removing the solids can, theoretically, be recovered.
In a thermal desalination system the “heat of vaporization” can be recovered in the condensation stage, referred to as the “heat of condensation.”
The first law of thermodynamics states that the total energy of an isolated system is constant; energy can be transformed from one form to another, but cannot be created or destroyed.
For a thermal process to be effective the system must be isolated (insulated) so that minimum heat energy escapes the system. The thermal process does not require energy form changes and can extract dry solids from seawater.
For a filtration process to recover and reuse the energy would require transforming from one form of energy to another (e.g., electrical to pressure) resulting in high entropy. The process cannot extract dry solids from seawater.
Therefore, there is still a need to create an efficient desalination that results in operational cost equal to, or less than, conventional ground water supplies.